Fuel delivery control systems, more particularly for turbojet engines

ABSTRACT

996,001. Gas turbine engine fuel systems. PLESSEY CO. Ltd. Oct. 12, 1961 [Oct. 25, 1960], No. 36577/60. Heading F1G. A fuel system for an engine such as a gas turbine jet engine comprises an engine-driven positive-displacement fuel supply pump, an adjustable throttle through which fuel passes on its way from the pump to the combustion chamber of the engine, means being provided for maintaining the pressure drop across the throttle constant by diverting the excess amount of fuel supplied by the pump to a point at low pressure. The system shown comprises a gear type main fuel pump 4 which receives fuel from a tank by way of a centrifugal type backing pump 65, the main pump delivering fuel through a throttle and spill valve unit 5 to a starting fuel burner 2 and a main burner 1. The throttle valve comprises a valve member 7 disposed within a sleeve 8 the valve member being adjusted by means of a control lever 6 through a rack and pinion drive 9. The valve member 7 is provided with tapered grooves 10 which co-operate with port 11 to which fuel is delivered by the pump 4 through line 34. The valve member 7 is shown in the position corresponding to the normal maximum rating of the engine in which fuel flows past the pressurising valve 26 to the main burner 1, fuel supply to the starting burner being cut off by the land 19 of the valve member. The spill valve 12 controls discharge of fuel through a dump port 18, the valve member being urged in the closing direction by means of a spring 13 and being controlled also by means of a diaphragm 14 which is subject to the pressure drop across the fuel metering orifice 10, 11, the space above the diaphragm being subject to fuel pressure upstream of the orifice by way of duct 15, and the space below the diaphragm being subject to fuel pressure downstream of the orifice by way of a duct 16 extending through the throttle valve member 7. If fuel pressure in the line 34 rises above a pre-determined value, the valve 12 opens and excess fuel is discharged through port 18. In the upper or shut-off position of the valve member 7 the port 11 is closed off by the land 19 and the space above the valve member 7 communicates with a dump line 21 by way of ducts 16, 20. As the valve member 7 is opened the annular recess 22 first opens communication between the port 11 and the port 23 whereby fuel flows to the starting jet 2. Continued opening movement of the valve member 7 attains an idling position in which the grooves 10 open to allow fuel to pass to the main burner 1 and simultaneously the supply to the starting burner 2 is cut off by the land 19. Also, the longitudinal port 24 establishes communication between the signal line 25 and the dump port 18. Further movement of the valve member allows increased fuel flow to the main burner 1, maximum normal flow being provided when the valve reaches the position shown. Under emergency conditions however, the valve member may be moved downwardly beyond the position shown, increasing flow through the metering orifice 10, 11. At the same time, the passage 24 is disconnected from the dump passage 18, but the duct 20 now communicates with the signal line 25 whereby the fuel pressure downstream of the metering orifice 10, 11 is communicated to line 25. The maximum-speed control of the fuel supply is effected by a governor unit comprising centrifugal weights 28 pivotally mounted on a sleeve 29, the weights acting through a rod 31 to control movement of a piston type slide valve 32 so as to progressively uncover ports 33 whereby fuel from the supply line 34 passes to a chamber 35 which communicates by way of duct 36 with the suction side of the main fuel pump 4. The opening action of the governor weights is assisted by a pair of springs 37, 38 but is resisted by a spring 63 acting on the valve 32. One end of a pivoted lever 39 is disposed between the adjacent ends of the springs 37, 38, the other end of the lever bearing on a piston 41. In the normal speed range of the engine, the space to the left of the piston 41 communicates by way of line 25 with the dump outlet 18 and no pressure acts on the piston. The governor weights allow excess fuel to pass through ports 33 to chamber 35 when the engine reaches its normal maximum speed. If however the control lever 6 is moved to the emergency position, the main burner pressure is communicated to the left-hand side of the piston 41 by way of the line 25. The piston therefore acts to move the lever 39 in a clockwise direction so compressing spring 37 and reducing the loading of the spring 38. The spring 63 will therefore move the piston valve 32 to close off the ports 33 so preventing escape of fuel from the line 34 to chamber 35 and the ports will only be re-opened when the emergency maximum speed of the engine has been reached. The system also comprises an acceleration control unit 45, an adjustable restriction 46 being disposed in the fuel supply line 34, the pressure upstream of the restriction being applied to a thrust member 51, and the pressure downstream of the restriction being applied to a piston valve member 47. The thrust member 51 is additionally loaded towards the right by a spring 52 and transmits its thrust to piston valve 47 by means of a thrust bearing unit 53. The bore in which the piston valve is disposed has two ports 54, 55, the port 54 communicating with the duct 34 and the port 55 communicating with a chamber 56 at one end of a freely movable link valve 57, the link valve having ports 59 by which fuel may pass from the chamber 56 to the chamber 35 which communicates with the suction side of the main fuel pump 4. The opposite side of the link valve is subject to the pressure in the supply duct 34 and so the link valve takes up a position such that the pressure in the chamber 56 is maintained equal to that in the duct 34. The pressure drop across the ports 54, 55 will thus be equal and therefore the ratio at which the fuel delivered by the pump 4 is distributed between duct 34 and the return to the pump inlet through the port 55 corresponds to the relative size of the apertures provided by the co-operation of the ports 54, 55 with their complementary apertures in the valve member 47. The pressure drop across the restrictor 46 and thus the displacement of the valve 47 will be a pre-determined function of engine speed. The valve member 47 and the link valve are continuously rotated by an engine-driven shaft 50, the governor sleeve 29 also being driven from the shaft 50, the gear 58 on the link valve acting as an intermediate gear.

April 13, 1965 H. G. TURNER FUEL DELIVERY CONTROL SYSTEMS, MORE PARTICULARLY FOR TURBOJET ENGINES Filed Oct. 16, 1961 United States Patent 3,177 $21 FUEL DELIVERY CDNTROL SYSTEMS, MORE PAR- TICULARLY FOR TURBOJET ENGINES Horace George Turner, Hornchurch, England, assignor to The Plessey Company Limited, llford, England, a British company Filed Oct. 16, 1961, Ser. No. 145,510 Claims priority, applicatioig grieat Britain, Get. 25, 1%0,

3 Claims. (Cl. 158-364) This invention relates to the control of a fuel supply to an engine such as a turbojet engine in which the fuel is supplied by a gear pump or other positive-displacement pump, and it has for an object to provide an improved control system in which the rate of fuel supply can be continuously varied in such manner that at each setting of the variable element the rate of fuel delivery is kept substantially constant, independently of variations of delivery pressure.

According to the present invention fuel is supplied to the combustion chamber of the turbine through an adjustable throttle, means being provided for maintaining the pressure drop across the throttle constant by divert ing the excess amount of fuel supplied by the pump to a point at low pressure.

According to a subsidiary feature of the invention additional means are preferably provided for limiting the amount of fuel supplied to the throttle controlling the delivery to the combustion chamber when the speed of the turbine approaches or exceeds'a preset maximum, and preferably also for limiting the flow of fuel supplied to the throttle in accordance with the turbine speed at lower speeds of the turbine to prevent fuel from being delivered to the engine at a greater rate than the maximum that at the prevailing engine speed can be usefully consumed. Preferably the first-mentioned limitation is arranged to become selectively operable at a normal maximum speed when the throttle control element moves within a normal operating range and at an emergency maximum speed when the control element is moved to an emergency position.

Other aspects of the invention consist in advantageous structural forms of the various control elements employed,'more particularly of a throttle and by-pass unit and of a maximum speed unit.

In order that the invention may be more readily understood, one embodiment will now be described with reference to the drawing acompanying the specification. The drawing is a somewhat diagrammatic sectional ele vation of the operating parts, of a fuel supply system for a turbojet engine, more particularly for a light turbojet engine for lifting purposes and also shows their hydraulic interconnection. In such engines, since the weight of aircraft does not appreciably vary with its height above ground, the thrust developed by the engine, and therefore the rate of fuel supplied to the engine, should also be substantially independent of height and therefore of atmospheric pressure and of the ambient temperature. The engine is equipped with a main burner 1 and a starting jet 2, to which 'fuel from a tank connection 3 is selectively supplied by a gear-type positive-displacement fuelpump The inlet pressure of this pump is maintained at a level reducing cavitation risk by the interposition of a centrifugal-type so-called backing pump between the tank connection 3 and the pump 4. The pump 4 is so dimensioned as to supply in all circumstances fuel at V a rate which is in excess of the maximum consumption of the engine. The amount of fuel delivered to the engine and its distribution to the main burner 1 or its starting jet 2 is controlled by a throttleand spill-valve unit 5 under the control of an engine-control level 6.

The throttleand spill-valve unit 5 essentially comprises a slide member 7 movable along a ported sleeve 8 by control lever 6 through a rack and-pinion drive 9. The slide member 7 has tapered control slots 10 adapted to cooperate with a port 11 to which fuel is supplied by the pump 4. A spill valve 12, connected to the line leading to port 11, is urged to its closed position by a spring 13 and is adapted to be opened against the pressure of the spring 13 by a diaphragm 14 on one side of which the pressure in port 11 is admitted through a passage 15, While the other side of the diaphragm communicates, through an axial bore 16 of the slide valve 7, with the pressure of the fuel delivered by that slide valve to the main burner 1. The spring 13 is set to a fixed value by means of an adjustment screw 17, and when the pressure difference between port 11 and bore 16 tends to rise above this value, spill valve 12 will open against the action of spring 13 to allow the excess fuel to escape through a dump port 18.

The slide valve 7, which is shown in its position corresponding to the normal maximum rating of the engine, serves to control the flow to the engine under all conditions. It has a shut-off position, in which a land 19 of the member 7 cuts oif port 11 from that end of the sleeve 8 which communicates with the main burner 1, and in which a branch 26) of the axial bore 16 in valve 7 opens to a dump line 21, thus ensuring that any leakage of fuel past the land 19 will not build up a pressure sulficient to deliver fuel to the main burner 1. As the slide 7 is moved progressively from the shut-01f position towards the illustrated maximum-rating position, it first arrives at a starting position, in which the dump connection 20, 21 is cut off, and in which an annular port 22 establishes connection between the admission port 11 and a starting port 23 leading to the starting jet 2. On further movement in the same direction the slide valve reaches an.

idling POSlllOlLiJJ. which the grooves 10 open a restricted path for the flow of fuel from port 11 to the main burner 1, while supply to the starting jet 2 is cut off by the land 19; at the same time a longitudinally extending port 24 in the slide member 7 establishes connection between the dump port 18 and a signal line 25 serving a purpose to be described further below. Further movement of slide member 7 in the same direction progressively increases the cr oss-section of the throttle passages provided by the slots 16 to allow increased flow from port 11 to the main burner 1, until the illustrated position and the maximum normal flow to the burner is reached. Under emergency conditions movement of control lever 6 and slide member 7 beyond this position is however per-- missible, which still further increases the available cross-. section at slots 1% and which at the same time disconnects the longitudinal passage 24 from dump 18 and instead establishes communication of branch 20 of axial passage 16 with the signal line 25, which thus is nowplaced under burner pressure instead of being under low pressure,

In order to ensure sufficient pressure build-up in the starting position to operate the starting jet, a spring-loaded pressurising valve 26 containing a restriction 27 is inter posed between the end of the sleeve 8 and the main 5 speed in-a manner to be described further below. The

weights 28 will, under centrifugal action, act on a rod 31 3,177,921 Patented Apr. 13, 1965 a a to urge a piston-type slide valve 32 against the action of a loading spring 63 to progressively uncover ports 33. These ports then connect the fuel supply line 34 leading to the throttleand spill valve unit being supplied with fuel by the pump 4, to achamber 35 which communicates through a line 36 with the suction side of the fuel pump 4. p The action of the governor weights 28 is supplemented by a pair of springs 37, 38, between which one end of a lever 39 pivoted at 40 is interposed. The other end of lever 39 bears on a signal piston 41 and normally holds that piston in contact with an adjustment screw 42.

The maximum-speed unit is illustrated in the position which it will assume when the throttleand spill-valve unit is set for a speed in the normal range so that the signal line communicates with the dump outlet 18, and no pressure therefore acts upon piston 41. Under these conditions the governor weights 28 will allow excess fuel to escape through ports 33 when the engine reaches its normal maximum speed. If however the control level 6 on unit 5 is moved to the emergency position, and accordingly signal line 25 is under main-burner pressure, piston 41 will move away from the end of the screw 42 until a stop shoulder 43 in piston 41 strikes an end flange 44 of the screw 42, thereby compressing spring 37 and reducing the pressure of the spring 38, i.e., thespring which assists the centrifugal action of the governor weights against the preloaded spring 63. As a result the spring 63 will still retain slide valve 32 so that no fuel will be allowed to escape at the normal maximum speed, and

'the slide valve 32 will only begin to clear ports 33 when the emergency maximum speed of the engine has been reached.

Conveniently the rod 31 and piston valve 32 are arranged to rotate with the governor weights 28, a ball thrust bearing (not shown) being interposed between the valve and rod on the one hand and the loading springs on the other hand. Such rotation of the valve 32 reduces to a minimum friction resistance against longitudinal movement; of the valve in response to the action of the governor weights.

The system further comprises an acceleration-control unit 45 which at engine speeds below the normal maximum speed limits the amountof fuel that can be supplied via the throttleand spill-valve unit 5 to that preventing compressor surge at the speed in question. For this purpose a preferably adjustable restrictor 46 is arranged in the delivery line of the fuel pump 4, and the pressures at the two sides of this restrictor are respectively applied to one end surface of a piston valve member 47 and to a thrust member 51. The valve member 47 slides in a ported bore 48. It is biased by a spring 47a opposing the upstream pressure and has an integral gear member 49 which, in order to reduce friction to longitudinal sliding movement of the member 47, is rotated by an enginedriven shaft 50 having appropriate elongated pinionteeth. The non-rotary thrust member 51 is loaded in the opposite direction by an adjustable spring 52 and transmits its thrust to piston member 47 via a thrust-bearing unit 53. The bore 48 has two ports 54 and 55, of which the former communicates with the line 34 leading to the throttleand spill-valve unit 5 While the latter leads via a passage 66 to a chamber 56 facing one end of a freely movable link valve 57. The latter is likewise rotated by the enginedriven'shaft 50 by means of a gear 58 integral with the link valve 57 and also acting as an intermediate gear transmitting the rotary movement from shaft 50 to the abovementioned gear on the governor unit. The link valve 57 has ports 59 communicating with chamber 56 and arranged, according to the position of the link valve 57, to divert a variable part of the fuel which is admitted through port 55, to chamber which, as previously mentioned, communicates with the suction side of the will therefore automatically assume such a position that the exposed part of ports 59is just sufiicient to maintain the pressure in chamber 56, and thus in line 66 and port 55, equal to the pressure in line 34. In this manner it is ensured that the pressure drop in ports 54 and 55 will be fore will rotate at a speed proportional to engine speed,

fuel pump 4. The opposite side of valve 57 is exposed with the result that the fuel flow through restrictor 46 will also be substantially proportional to engine speed, and that the pressure drop across the restrictor 46, and thus the displacement of the valve 47, will be a predetermined function of the engine speed. By suitably shaping the apertures of ports 54 and 55 and/or the apertures 54a and 55a in the slide valve 47 co-operating with the former, this ratio can be determined accurately for each engine speed in accordance with the characteristics of the engine, so that at each speed the maximum flow available at main burner 1 is limited to the amount which at the actual engine speed will produce the maximum torque.

It will be appreciated that various details of the embodiment described with reference to the accompanying drawing may be modified without exceeding the scope of the present invention. More particularly the acceleration control unit may be replaced by another unit similarly limiting the maximum fuel supply available in accordance with maximum fuel requirements of the engine for any given speed.

What we claim is:

1. A fuel-supply system for turbojet engines and other combustion engines having a combustion chamber, the system comprising an engine-driven positive-displacement pump delivering fuel at a rate proportional to the speed of the combustion engine, passage means leading from said pump to the combustion chamber and including an adjustable restrictor and an adjustable throttle valve arranged downstream of the restrictor so that fuel on its way from the pump to the combustion chamber passesfirst through the restrictor and then through'the throttle, a by-pass connected to said passage means between the restrictor and throttle and leading to a low-pressure point, a b y-pass valve in said by-pass, having actuating means acted-upon by the difference of the pressures upstream and downstream of the throttle to open said by-pass valve against a resilient loading, a valve housing having a bore, a slide valve member axiallymovable in said bore and including a portion constituting said throttle valve and operative to increase the throttle aperture progressively when moved from a first to a second position, a signal port in the valve housing, communicating with said bore the slide valve member including control means for the signal port operative to connect said port with a point under a first pressure when the slide valve is between said first and second positions and to isolate said port from said first pressure and connect it with a point under a second pressure when the slide valve is moved beyond said second position, a spill passage connecting a point between said restrictor and throttle of said passage means with a low-pressure point, a spill valve in said spill passage, a centrifugal governor driven by the engine and operative under centrifugal action against resilient loading to open said spill valve at a predetermined engine speed, a piston member operatively connected to said governor through a resilient means, said piston member beingsubject to the pressure. at said signal port to effectively increase said resilient loading when the pressure at said signal port changes. from said first to said second value, an acceleration-control slide valve housing having an inlet communicating with the said passage means at a point between the restrictor and the point leading to said by-pass and having a first outlet leading to said throttle and a second outlet leading to a spill passage, a distributor slide-valve element movable in said slidevalve housing to increase the effective aperture of one of said outlets and decrease that of the other said outlet or vice versa, an acceleration-control piston member at least operatively connected to said slide valve element, said piston member hing subjected to the difference of the pressures at the two sides of the restrictor acting, against a resilient loading means, to increase the first and decrease the second outlet when the said difierence increases, a balanced slide-valve throttle in the connection from the second outlet to the spill passage, said slidevalve throttle having opposed piston surfaces, one of said surfaces subject to the pressure between the first outlet and the control throttle urging the slide-valve throttle to reduce its throttle aperture and the other surface subject to the pressure between the second outlet and the slidevalve throttle to oppose the said aperture-reducing action.

2. A fuel-supply system for turbojet engines and other combustion engines having a combustion chamber, the system comprising an engine-driven positive-displacement pump delivering fuel at a rate proportional to the speed of the combustion engine, passage means leading from said pump to the combustion chamber and including an adjustable throttle valve through which the fuel passes on its way from the pump to the combustion chamber, a bypass connected to said passage between the pump and throttle and leading to a low-pressure point, a by-pass valve in said by-pass, having actuating means acted-upon by the difference of the pressures upstream and downstream of the throttle to open said by-pass valve against a resilient loading, a valve housing having a bore, a slide valve member axially movable in said bore and including a portion constituting said throttle valve and operative to increase the throttle aperture progressively when moved from a first to a second position, a signal port in the valve housing communicating with said bore the slide valve member including control means for the signal port operative to connect said port with a point under a first pressure when the slide valve is between said first and second positions and to isolate said port from said first pressure and connect it with a point under a second pressure when the slide valve is moved beyond said second position, a spill passage connecting a point, between said pump and said throttle, of said passage means with a low-pressure point, a spill valve in said spill passage, a centrifugal governor driven by the engine and operative under centrifugal action against resilient loading to open said spill valve at a predetermined engine speed, and a piston member operatively connected to said governor through a resilient means, said piston member being subject to the pressure at said signal port to effectively increase said resilient loading when the pressure at said signal port changes from said first to said second value.

3. A fuel-supply system for turbojet engines and other combustion engines having a combustion chamber, the system comprising a positive-displacement fuel pump, passage means leading from said pump to the combustion chamber adjustable means controlling the amount of fuel thus delivered to the combustion chamber, an adjustment element for said control means said adjustment element being manually movable from a low-power position through a normal control range to a high-power position and to an emergency position beyond said high-power position, a spill passage connecting a point of said passage means with a low-pressure point, a signal line, an emergency control device for the signal line, said device being operatively connected to said adjustment element to connect said signal line with a point under a first pressure when the adjustment element is in the normal control range and to isolate said signal line from said first pressure and connect it with a point under a second pressure when the adjustment element is moved beyond said range to the emergency position, a spill passage connecting a point of said passage means with a low-pressure point, a spill valve in said spill passage, a centrifugal governor driven by the engine and operative under centrifugal action against resilient loading to open said spill valve at a predetermined engine speed, and a piston member opera-tively connected to said speed-control spill valve through a resilient means, said piston member being subject to the pressure in said signal line to effectively increase said resilient loading when the pressure in said signal line changes from said first to said second value.

References Cited by the Examiner UNITED STATES PATENTS W erts.

OTHER REFERENCES Spray Spill-Burner Fuel System in Aircraft Engineermg, volume XXV, No. 29 1, pp. 133-139 of May 1953.

JAMES W. WESTHAVER, Primary Examiner.

PERCY L. PATRICK, FREDERICK KETTERER,

' Examiners. 

1. A FUEL-SUPPLY SYSTEM FOR TURBOJET ENGINES AND OTHER COMBUSTION ENGINES HAVING A COMBUSTION CHAMBER, THE SYSTEM COMPRISING AN ENGINE-DRIVEN POSITIVE-DISPLACEMENT PUMP DELIVERING FUEL AT A RATE PROPORTIONAL TO THE SPEED OF THE COMBUSTION ENGINE, PASSAGE MEANS LEADING FROM SAID PUMP TO THE COMBUSTION CHAMBER AND INCLUDING AN ADJUSTABLE RESTRICTOR AND AN ADJUSTABLE THROTTLE VALVE ARRANGED DOWNSTREAM OF THE RESTRICTOR SO THAT FUEL ON ITS WAY FROM THE PUMP TO THE COMBUSTION CHAMBER PASSES FIRST THROUGH THE RESTRICTOR AND THEN THROUGH THE THROTTLE, A BY-PASS CONNECTED TO SAID PASSAGE MEANS BETWEEN THE RESTRICTOR AND THROTTLE AND LEADING TO A LOW-PRESSURE POINT, A BY-PASS VALVE IN SAID BY-PASS, HAVING ACTUATING MEANS ACTED-UPON BY THE DIFFERENCE OF THE PRESSURES UPSTREAM AND DOWNSTREAM OF THE THROTTLE TO OPEN SAID BY-PASS VALVE AGAINST A RESILIENT LOADING, A VALVE HOUSING HAVING A BORE, A SLIDE MEMBER AXIALLY MOVABLE IN SAID BORE AND INCLUDING A PORTION CONSTITUTING SAID THROTTLE VALVE AND OPERATIVE TO INCREASE THE THROTTLE APERTURE PROGRESSIVELY WHEN MOVED FROM A FIRST TO A SECOND POSITION, A SIGNAL PORT IN THE VALVE HOUSING, COMMUNICATING WITH SAID BORE THE SLIDE VALVE MEMBER INCLUDING CONTROL MEANS FOR THE SIGNAL PORT OPERATIVE TO CONNECT SAID PORT WITH A POINT UNDER A FIRST PRESSURE WHEN THE SLIDE VALVE IS BETWEEN SAID FIRST AND SECOND POSITIONS AND TO ISOLATE SAID PORT FROM SAID FIRST PRESSURE AND CONNECT IT WITH A POINT UNDER A SECOND PRESSURE WHEN THE SLIDE VALVE IS MOVED BEYOND SAID SECOND POSITION, A SPILL PASSAGE CONNECTING A POINT BETWEEN SAID RESTRICTOR AND THROTTLE OF SAID PASSAGE MEANS WITH A LOW-PRESSURE POINT, A SPILL VALVE IN SAID SPILL PASSAGE, A CONTRIFUGAL GOVERNOR DRIVEN BY THE ENGINE AND OPERATIVE UNDER CENTRIFUGAL ACTION AGAINST RESILIENT LOADING TO OPEN SAID SPILL VALVE AT A PREDETERMINED ENGING SPEED, A PISTON MEMBER OPERATIVELY CONNECTED TO SAID GOVERNOR THROUGH A RESILIENT MEANS, SAID PISTON MEMBER BEING SUBJECT TO THE PRESSURE AT SAID SIGNAL PORT TO EFFECTIVELY INCREASE SAID RESILIENT LOADING WHEN THE PRESSURE AT SAID SIGNAL PORT CHANGES FROM SAID FIRST TO SAID SECOND VALUE, AN ACCELERATION-CONTROL SLIDE VALVE HOUSING HAVING AN INLET COMMUNICATING WITH THE SAID PASSAGE MEANS AT A POINT BETWEEN THE RESTRICTOR AND THE POINT LEADING TO SAID BY-PASS AND HAVING A FIRST OUTLET LEADING TO SAID THROTTLE AND A SECOND OUTLET LEADING TO A SPILL PASSAGE, A DISTRIBUTOR SLIDE-VALVE ELEMENT MOVABLE IN SAID SLIDEVALVE HOUSING TO INCREASE THE EFFECTIVE APERTURE OF ONE OF SAID OUTLETS AND DECREASE THAT OF THE OTHER SAID OUTLET OR VICE VERSA, AN ACCELERATION-CONTROL PISTON MEMBER AT LEAST OPERATIVELY CONNECTED TO SAID SLIDE VALVE ELEMENT, SAID PISTON MEMBER BEING SUBJECTED TO THE DIFFERENCE OF THE PRESSURE AT THE TWO SIDES OF THE RESTRICTOR ACTING, AGAINST A RESILIENT LOADING MEANS, TO INCREASE THE FIRST AND DECREASE THE SECOND OUTLET WHEN THE SAID DIFFERENCE INCREASES, A BALANCED SLIDE-VALVE THROTTLE IN THE CONNECTION FROM THE SECOND OUTLET TO THE SPILL PASSAGE, SAID SLIDEVALVE THROTTLE HAVING OPPOSED PISTON SURFACES, ONE OF SAID SURFACES SUBJECT TO THE PRESSURE BETWEEN THE FIRST OUTLET AND THE CONTROL THROTTLE URGING THE SLIDE-VALVE THROTTLE TO REDUCE ITS THROTTLE APERTURE AND THE OTHER SURFACE SUBJECT TO THE PRESSURE BETWEEN THE SECOND OUTLET AND THE SLIDEVALVE THROTTLE TO OPPOSE THE SAID APERTURE-REDUCING ACTION. 